Continuous Casting Process
Continuous casting, in the steel-making industry, is the process whereby molten steel is solidified into a semifinished product such as a billet, bloom, or slab for subsequent rolling in a hot strip mill or a finishing mill. This process is achieved through a well-designed casting machine, known as a continuous caster.
FIG. 1 shows a schematic diagram of a continuous caster according to the prior art, which comprises the following key elements: a ladle turret 20, a ladle 22, a tundish 24 with a stopper-rod 26, a submerged entry nozzle (SEN) 28, a water-cooled copper mold 30, a roller containment section with additional cooling chambers 32, a straightener withdrawal unit 34 and torch severing equipment 36.
Molten steel from an electric arc furnace or a basic oxygen furnace is tapped into a ladle and shipped to the continuous caster. This batch of steel, referred to as a heat, will be used to cast several slabs, blooms or billets. The ladle is placed into the casting position above the tundish 24 by the turret 20. The steel is poured into the tundish 24, and then into the water-cooled copper mold 30 through the SEN 28, which is used to regulate the steel flow rate and provide precise control of the steel level 38 in the mold. As the molten steel moves down the mold 30 at a controlled rate (referred to as casting speed), the outer shell 39 of the steel becomes solidified to produce a steel strand 40. Upon exiting the mold 30, the strand 40 enters a roller containment section and cooling chamber in which the solidifying strand is sprayed with water to promote solidification. Once the strand is fully solidified and has passed through the straightener withdrawal unit 34, it is cut to the required length by the severing unit 36 and becomes a slab 46.
Special Casting Practice
Referring to the casting speed profile 50 shown in FIG. 2, the entire operation sequence of a continuous caster consists of a brief start-up operation 52, followed by a prolonged continuous, run-time production operation 54, and finally a shut-down operation 56. The main operational issues in continuous casting processes relate to achieving a stable operation following start-up, and then maintaining stability. In order to improve casting process efficiency and flexibility, it is expected to continuously cast as many heats as possible in one single casting sequence. For this purpose, certain special casting practices need to be performed during the continuous, run-time production operation. For example, SEN changes and flying tundish changes are activated to replace an SEN or tundish in current operation when their service time expires; product grade changes require the insertion of a product grade separator so as to cast different grades of steel as successive heats. All these special casting practices, referred to as transient operations in this invention, require the casting speed to be decreased. The resulting process trajectories share a common feature that, during an SEN change, a flying tundish change or a product grade change, the casting speed is first slowed down considerably (approximately 0.6 meters/minute for an SEN change and 0.1 meters/minute for a flying tundish change or product grade change); then it remains unchanged for a short period of time, during which the consumed SEN/tundish is replaced or a product grade separator is inserted, in an automatic or manual fashion; and finally the casting speed is ramped up gradually back to its normal operating conditions over several minutes. The effect on the casting speed profile during an SEN change 58 and a flying tundish change 60 are shown in FIG. 2. It is worth noting that any improper transient operations may increase risks of damaging the steel strand and causing a catastrophic breakout as described below.
Breakout and its Prevention
A well-known problem associated with the continuous caster, is that solidifying steel is prone to tears in the strand shell 39FIG. 1 due to a variety of causes including friction, inclusion, insufficient or imperfect solidification, etc. and this causes a breakout such that molten steel breaks out of the strand shell immediately beneath the mold, resulting in an emergency stop cast. A breakout may occur during a start-up operation, known as a start-cast breakout, or during the following run-time operation, known as a run-cast breakout, or during one of the aforementioned transient operations, known as a transient-cast breakout. Based on some statistics of plant operations, for a typical, fully operational continuous caster, more than 50% of total breakouts are due to improper transient operations such as SEN changes, flying tundish changes, etc. These breakouts are of major concern in the steel-making industry, because they diminish the reliability and efficiency of the production process, create substantial costs due to production delays and destruction of equipment, and most importantly, pose significant safety risks to plant operators. Therefore, the ability to prevent breakouts from happening utilizing engineering expertise and analytical methods can provide excellent benefits to the continuous casting process.
Although there have already been some methods and systems developed to predict run-cast breakouts in the prior art, breakouts occurring in transient operations and their prevention has received very little attention in both academia and industry. It is important, then, to be able to predict these breakouts in advance such that they can be avoided by taking appropriate control actions.
According to the prior art in the area of predicting breakouts in continuous casting processes, there are two different types of methods. One is the pattern-matching method, for example, the well-known sticker detection method, which develops comprehensive rules to characterize the patterns in the mold C, temperatures prior to the incidence of a breakout based on past casting operation experiences. If such patterns are recognized in the current casting operation, then there is a high likelihood that a breakout will occur. The relevant systems based on this type of method are described by Yamamoto et al in U.S. Pat. No. 4,556,099, Blazek et al in U.S. Pat. No. 5,020,585, Nakamura et al in U.S. Pat. No. 5,548,520, and by Adamy in U.S. Pat. No. 5,904,202. In addition, following the similar philosophy of the pattern-matching method, Frtiz-Peter Pleschiutschnigg described a method in U.S. Pat. No. 6,179,041 B1 for continuous caster breakout early-reorganization, which uses a comparison of oscillation measurements with breakout-relevant signals to recognize a breakout tendency. The other method is a multivariate statistical method described by Vaculik et al in U.S. Pat. No. 6,564,119, where a principal component analysis (PCA) model is built using an extended set of process measurements, beyond the standard mold temperatures, to model the normal operation of casting processes; certain statistics are then calculated by the model to detect exceptions to normal operation in the current casting operation and to predict potential breakouts. Both of these methods, however, are focused on predicting the run-cast breakouts, and won't work due to some technical difficulties when they are applied to the transient-cast breakouts. The biggest obstacle for these methods is that they are not able to deal with significant changes of process dynamics during transient operations.
Multivariate SPC for Batch Processes
The applicant is also aware of prior art in the use of multivariate statistical process control (SPC) technology for batch process monitoring and fault diagnosis in other fields. Examples of methods and industrial applications of monitoring a batch process using multivariate SPC technology are described by MacGregor and his co-workers in AIChE Journal, volume 40, 1994, Journal of Process Control, volume 5, 1995. There is no application of such multivariate SPC technology to continuous caster transient operations described in the patent literature.